1. Field of the Invention
The present invention relates to a hydraulic circuit for a hydraulic power transmission such as a fluid coupling or a torque converter, and more particularly, relates to control of supply of hydraulic pressure to a hydraulic power transmission provided with a lock-up clutch.
2. Description of the Related Art
Hydraulic power transmissions generally have a pump impeller and a turbine runner disposed opposite each other within a hydraulic power transmission chamber, and a lock-up clutch disposed therebetween for directly connecting the pump impeller to the turbine runner. Such an arrangement is intended to improve torque transmission efficiency in an area where torque-transmitting fluid has low slippage. Lock-up clutches can be roughly divided into two types. In one type of lock-up clutch, a flexible single-disc clutch fixed to a turbine hub engages and disengages the case of the hydraulic power transmission. In the other type, a multi-disc friction engagement element is disposed between the turbine runner and the case of the hydraulic power transmission. The friction engagement element is engaged and disengaged by operation of a servo piston disposed within a servo oil chamber that is provided isolated from the hydraulic power transmission chamber within the hydraulic power transmission case.
The single-disc type of clutch establishes a lock-up OFF state when oil is supplied to the hydraulic power transmission chamber from an oil passage (a lock-up OFF oil passage), between the clutch plate and the hydraulic power transmission case, and the oil is discharged from a lock-up ON oil passage. Conversely, a lock-up state is established when the lock-up clutch is engaged by supply of hydraulic pressure to the hydraulic power transmission chamber side from the lock-up ON oil passage. The hydraulic pressure in this type of hydraulic power transmission is supplied via two oil passages that alternately feed and discharge hydraulic pressure, namely, the lock-up ON oil passage and the lock-up OFF oil passage.
However, a simple lock-up control such as performed for the single-disc clutch cannot be applied in the other type, wherein a multi-disc friction engagement element is engaged and disengaged by a servo piston, because the servo piston is pushed inside the servo oil chamber when hydraulic pressure is supplied to the hydraulic power transmission chamber. For this reason, the supply of hydraulic pressure to the hydraulic power transmission in the related art is achieved by two oil passages that alternately feed and discharge hydraulic pressure, namely, an oil passage that supplies oil to an operating chamber and an oil passage that discharges oil from within the operating chamber. A separate oil passage is provided exclusively for lock-up control that controls the supply of hydraulic pressure to the servo oil chamber (refer to Japanese Patent No. 2641419).
The single-disc type of lock-up clutch has a simple structure and needs only two oil feed passages, which makes it advantageous in terms of circuit configuration. However, a corresponding increase in the clutch outer diameter is necessary in order to handle cases of greater torque. On the contrary, the multi-disc type of lock-up clutch is capable of transmitting torque through engagement of a number of friction plates, making it advantageous in terms of its capability to transmit a greater torque, while avoiding an increase in the size of the clutch outer diameter. However, the structure is complex and a complicated circuit configuration is unavoidable due to the necessity for three oil feed passages as mentioned above.
Furthermore, in the hydraulic circuit of the related art, it is necessary to dispose a control valve for controlling the torque converter, in addition to a control valve for controlling the lock-up clutch. This not only risks cost increases, but also requires an enlarged space for the circuit as well.